Helical wave guides



Dec. 1, 1959' .-A. YOUNG, JR 2,915,715

HELICAL WAVE GUIDES Filed July 20, 1956 MODE 1 L 730 TEN 22. If .80l.366 FIG. 3 24.5 .820 .593

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' lNl/E N TOR J- ,4. YOUNG, JR.

United States HELICAL WAVE GUIDES James A. Young, Jr., Fair Haven, N.J.,assignor to Bell Telephone Laboratories, Incorporated, New York, N .Y.,

This invention relates to electromagnetic wave transmission systemsemploying a wave guide of circular cross section and, more particularly,to mode and polarization selective elements for use in such systems.

It is an object of the present invention freely transmit a given roundguide mode of electromagnetic wave energy and to substantially attenuateall other modes.

It is another object of the invention to freely transmit a given senseof circular polarization of a round wave guide mode of electromagneticwave energy and to substantially attenuate the opposite sense ofcircular polarization.

' It has been known that electromagnetic waves propagating in aconductively bounded transmission medium of circular cross sectioninduceconduction currents in the cylindrical conductive boundary. Waves of thetransverse electric type, that is, waves having electric vectors whichare everywhere perpendicular to the direction of propagation, arecharacterized by the fact .that the induced wall currents in generalhave both longitudinal and transverse components, the longitudinalcomponent being parallel to the axis of propagation and the transversecomponent being perpendicular thereto. The resultant direction'of wallcurrent flow is therefore at some angle to the axis of propagation, thisangle corresponding to the proportion between the longitudinal andtransverse components. Furthermore, each such mode has a unique andditferent direction of wall current flow, depending upon the particularfield' configuration associated with that mode. p

In accordance with the present invention, an anisotropic conductingboundary is provided for a circular wave guiding path such thatunattenuated wall current conduction takes placeonlyv at one specificangle and substantial dissipation is introduced into curents flowing at.all other angles.

More particularly, it has been found that a wave guiding structurehaving a helically wound conductive boundary will provide asubstantially lossless transmission path for the modehaving a directionof wall current flow which lies along the helical windings.

At the same time, all other modes, having different directions of wallcurrent flow, will see as discontinuous wave conduction path and willbridge these discontinuities between windings in the form ofdisplacement currents. These displacement currents can be dissipated ina lossy sheath or jacket surrounding the helix. The wave guidingstructure thus provided acts as a mode filter in that it willpreferentially propagate only one mode and will severely attenuate allother modes. Furthermore, the sheathed helical wave guide is capable ofdistinguishing between opposite senses of circular polarization ofelectromagnetic wave energy. It will freely propagate only that sense ofcircular polarization corresponding to the circular sense of the helicalwinding and will dissipate wave energy having the opposite sense ofcircular'polarization. This helical structure can therefore be used as afilter for one sense of circular polarization or as a transducer fromlinearly polarized to cir- 2,915,715 Patented Dec. '1, 1959 electricfield vector at a point describes a circle in the plane of the crosssection as time progresses and does not change in amplitude. The lattershould not be confused with the circular electric or circular magneticwaves which have no direction of polarization. It is well known that alinearly polarized wave can be considered as two circularly polarizedwaves of the same frequency but of opposite circular polarizations.

These and other objects, the nature of the present invention and itsvarious advantages, will appear more fully upon consideration of thevarious specific illustrative em-' bodiments shown in the accompanyingdrawings and the following detailed description of these embodiments.

In the drawings:

Fig. 1 is a perspective view of a wave transmission system employing ahelical filter in accordance with the principles of the invention;

Fig. 2 is a partially cut away view of the helical filter shown in Fig.1;

Fig. 3 is a table of the helix pitch anglesand diameter ratios requiredto freely pass certain of the lower order modes; and

Fig. 4 is a partially cut away View of a multifilar helical filter inaccordance with the principles of the invention. Referring moreparticularly to Fig. 1, there is shown as a specific illustrativeembodiment of the invention an electromagnetic wave transmission systemcomprising a source 11 of microwave energy in some given transverseelectric mode, preferably the dominant TE mode for the round pipe waveguide, and having a given sense of circular polarization. Source 11 may,for example, supply TE mode energy in the clockwise or right hand senseof circular polarization, as viewed along the direction of propagation,corresponding to the direction of rotation of a right hand screw whenadvancing in the direction of propagation. Source 11 is connected toload 12 by means of wave guide transmission path 13 of substantiallycircular cross section. Source 11 and load 12 are adapted for the righthand sense of circular polarization of the dominant mode whiletransmission path 13 represents the type of transmission line havingbends, joints, slight physical imperfections, and deliberately insertedcomponents, any of which tend to introduce multimoding and asymmetricalreactance effects to the energy transmitted therealong. As is wellknown, these effects convert portions of the normally right handcircularly polarized dominant mode wave into other spurious modes andinto the opposite left hand sense of circular polarization, resulting inelliptically polarized wave components. To prevent undue reflectionsfrom load 12, these spurious modes and polarizations must be eliminatedin some manner prior to entering load 12.

In accordance with the present invention, the spurious modes andpolarizations introduced by path 13 are eliminated from the outputthereof by a filter 14 comprising a helically wound conductor 15surrounded and supported by a. lossy jacket 16. The walls of filter 14will conduct electrical currents in only one direction, corresponding tothe direction of the helical windings, and will dissipate modes andpolarizations having different directions of wall current flow. Theoperation of the filter may be better understood upon consideration ofFig. 2 which shows'filter 14 separated from the system shown in Fig. 1.

Referring therefore to Fig. 2, the axis 17 designates the axis ofpropagation of wave energy in filter 14. As shown in Fig. 2, the helixformed by conductor 15 has an inner diameter of 2a and a helical pitchangle of 1/, where the pitch angle is defined by 3 tan where s is thespacing between turns. Jacket 16 is composed of highly dissipatingmaterial for attenuating the displacement currents existing betweenadjacent turns of wire 15. Jacket 16 may, for example, comprise adielectric material such as polystyrene or Teflon in which carbonparticles are embedded. Conductor 15 has a diameter which is smallcompared to the wavelength of the energy in filter 14 and adjacent turnsof conductor 15 are closely spaced but electrically separated from oneanother. Conductor 15 may, for example, be an enamelcovered copper wirewhich is closely wound to form the helix. To arrive at the proper pitchangle ;J/, a multifilar winding as shown in Fig. 4, comprising aplurality of insulated conductive wires i.e., 18 and 19, may be usedinstead of single conductor 15 of Fig. 2, for it is evident that as thepitch increases, the space between adjacent turns will increase, thusexposing the preferred mode to the dissipative jacket 16. This may beavoided, however, by using a multifilar winding. It is then possible touse the large pitch angle called for by the mode to be propagated andstill satisfy the other two requirements of an etficient filter, namelythat the diameter of conductor 15 be small compared to the wavelength ofthe energy of the preferred mode to be transmitted through the filterand that adjacent turns of conductor be closely spaced. Thus, in Fig. 4,the space s between adjacent turns of winding 18 is filled in by a turnof Wire 19, thus providing a low loss path [or the wall currentsassociated with the preferred mode.

It can be shown from the characteristic equations for a helical waveguide such as that shown in Fig. 2 that the attenuation constant of sucha geometry for a transverse electric TE mode is approximately given by n1-v 3 2 im 1/2 where:

a=the attenuation constant of the helix,

K=a constant depending upon the properties of lossy jacket 16,

a=the inner radius of filter 14,

v=l =the ratio of free-space wavelength t to cutoff wavelength )t of theWaves in filter l4,

p the mth zero of the derivative of the Bessel function of order n,

m the number of the mode,

n the order of the mode,

r=the pitch angle of the helix,

and the plus sign denotes that sense of circular polarization of thetransverse electric wave opposite to the sense of the helical winding ofconductor 15, While the minus sign denotes that sense of circularpolarization the same as the sense of the helical winding.

It can be seen from a consideration of Equation 1 that the attenuationconstant of a helical wave guide can be made substantially equal to zerowhen the numerator of the quantity on the right hand side of the equalsign is made to equal zero. This condition is attained when thebracketed quantity is zero, which occurs when the sense of the helicalwinding is the same as the sense of the circular polarization of thewaves transmitted therethrough and the helix pitch angle is given by Ahelix having the above-defined pitch angle will theoretically have zeroattenuation (assuming infinite con ductivity of the helix) for the TEmode having a sense of circular polarization corresponding to the senseof the helical winding. In this case, the wall currents of this mode lieexactly along the helical windings and suffer little or no wallattenuation loss. hand, all of the other modes having wall currentswhich do not lie along the helix will be attenuated as displacementcurrents in lossy jacket '16. The transverse electric modes having asense of circular polarization opposite to the sense of the helicalwinding will also have wall current components which do not lie alonghelically wound conductor 15 and will also be attenuated by lossy jacket16. Furthermore, for a particular frequency of operation this loss tothe opposite sense of circular polarization can be made a maximum. Moreparticularly, when the opposite sense of circular polarization is mostseverely attenuated. The table in Fig. 3 of the drawings illustrates thevalues of 0 and 1 which will satisfy both Equation 2 and Equation 3 forthe difierent modes. Also given is the value of a/A corresponding to theparticular values of 1/. In view of the preceding explanation, it can beseen that a sheathed helix can be designed to pass any transverseelectric mode in any desired sense of circular polarization merely bychoosing the proper parameters. All other modes and polarizations willbe severely attenuated. For example, a helix has been constructed forthe dominant TE mode which will transmit one sense of circularpolarization of this mode with less than a .1 decibel loss whileattenuating the opposite sense of polarization more than 25 decibels.

As is evident from the foregoing comments, the structure shown in Fig. las filter 14 may also be used as a transducer from linearly polarized tocircularly polarized waves. A properly constructed helix of sufiicientlength will completely absorb the undesired circularly polarizedcomponent, leaving only the desired sense of polarization.

In all cases it is understood that the above-described arrangements areillustrative of a small number of the many possible specificillustrative embodiments which can represent applications of theprinciples of the invention. Numerous and various other arrangements canreadily be devised in accordance with these principles by those skilledin the art without departing from the spirit and scope of the invention.

What is claimed is:

1. In an electromagnetic wave transmission system, means for producingwave energy components having at least one sense of circularpolarization, means for utilizing a component of said energy in a givensense of circular polarization, filtering means connecting saidutilizing means to said producing means for selectively transmittingsaid wave energy in said given sense of circular polarization andattenuating all other modes and polarizations said filtering meanscomprising a plurality of elongated members of insulated conductivematerial wound in a substantially helical form, said members beingsubstantially aligned with the direction of wall current fiow for saidgiven sense of circular polarization whereby said given polarization isfreely transmitted, and energy dissipating means surrounding said helixwhereby said other modes and polarizations are attenuated.

2. The combination in claim 1 wherein the distance between and thediameter of said conductive members are small compared to a wavelengthof said Wave energy in said given sense of circular polarization.

3. In an electromagnetic wave transmission system, means for producingwave energy components having at least one sense of circularpolarization, means for utiliz ing a component of said energy in a givensense of circular polarization, filtering means connecting saidutilizing means to said producing means for selectively transmit On theother ting said wave energy in said given sense of circular polarizationcomprising a helically wound conductive member surrounded by anelectrically dissipating sheath, said member having the same sense ofwinding as said given sense of circular polarization, and in which saidmember has a pitch angle and diameter for a given frequency given by ofthe mode of said wave energy, p being the mth zero of the derivation ofthe Bessel function of order n, and 1 being the ratio of the free-spacewavelength to the cut-off wavelength of the frequency of said waveenergy.

References Cited in the file of this patent UNITED STATES PATENTS2,720,609 Bruck et a1. Oct. 11, 1955 2,746,018 Sichak May 15, 19562,779,006 Albersheim Ian. 22, 1957 2,848,696 Miller Aug. 19, 1958FOREIGN PATENTS Canada July 6, 1954

